The background art is described by an example of a slewing drive device for an excavator.
The excavator is provided with a crawler-type lower traveling body, an upper stewing body loaded on the lower traveling body to be slewable around an axis perpendicular to the ground, and a working attachment attached to the upper slewing body.
The slewing drive device for the excavator is configured to slew the upper slewing body. The stewing drive device is provided with a hydraulic motor or an electric motor as a drive source, and a speed reducer including a gear reduction mechanism for reducing a speed of a rotation of the motor to transmit the rotation whose speed is reduced by the gear reduction mechanism to the upper slewing body as a driven unit.
The motor and the speed reducer are arranged side by side along the axis direction of the slewing drive device in a state that the rotating axes thereof are aligned with each other. The motor and the speed reducer are mounted on an upper frame in a vertical posture such that the motor is disposed above the speed reducer.
The speed reducer is a planetary gear speed reducer including a sun gear, a plurality of planetary gears, and a ring gear disposed around the rotating axis of the speed reducer. The output of the speed reducer is transmitted to the upper slewing body, whereby the upper slewing body is slewed.
A general stewing drive device is configured as described above. Patent literature 1 discloses a slewing drive device provided with a brake mechanism in a speed reducer.
The speed reducer of the stewing drive device disclosed in patent literature 1 is provided with a plurality of speed reducing units arranged side by side along the axis direction of the rotating shaft of the speed reducer. The brake mechanism provided in the speed reducer includes a rotating-shaft-side brake plate mounted on the rotating shaft at a position between the speed reducing units adjacent to each other, a casing-side brake plate provided on the inner periphery of a casing, and a brake piston. The rotating-shaft-side brake plate and the casing-side brake plate are brought into press contact with each other by a pressing force of the brake piston, whereby a braking force is exerted on the rotating shaft of the speed reducer.
In the slewing drive device disclosed in patent literature 1, a brake mechanism is interposed between the speed reducing units adjacent to each other. This may increase the size of the speed reducer in the axis direction of the rotating shaft. As a result, the overall size of the slewing drive device in the axis direction may increase. In a construction machine such as an excavator, the entire length of the slewing drive device is restricted for a reason such that a hydraulic pipe is disposed above the slewing drive device. Therefore, an increase in the axial size of the slewing drive device as described above makes it difficult to mount the slewing drive device in the construction machine.
Further, in a disc brake mechanism for braking a rotating shaft by a frictional force between brake plates, there is a demand for increasing the distance (hereinafter, called as a brake effective radius) from the center of the rotating shaft to the point of friction between the brake plates as much as possible in order to enhance the braking performance. In order to satisfy the above demand, in patent literature 1, a disc-shaped brake connecting member is mounted on the outer periphery of the rotating shaft, and a rotating-shaft-side brake plate is mounted on the outer periphery of the brake connecting member, whereby the brake effective radius is increased.
In the above configuration, however, the number of components may be increased, the cost necessary for the brake mechanism may be increased, and the assembling performance of the brake mechanism may be deteriorated. There is an idea of increasing the brake effective radius in a state that the rotating-shaft-side brake plate is directly mounted on the rotating shaft by increasing the outer diameter of the rotating-shaft-side brake plate. Even in this configuration, however, the problems relating to the cost necessary for the brake mechanism, and the assembling performance of the brake mechanism are not solved.